The political and physical environment of larger cities makes decisions on heavy construction projects not always as clear an issue as in rural areas. Urban tunnelling has to be considered not as three-dimensional, not even as four-dimensional including time, but as five-dimensional including politics.

The issues confronting urban tunnelling today are not "can the tunnel technically be done?" but rather "what are the advantages and disadvantages of adopting one tunnelling technique over another, and the consequences of this decision?".